a. Field of the Invention
Broadly speaking, this invention relates to the compression or expansion of electrical signals. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for compressing or expanding electrical signals by the use of magnetic storage devices.
B. Summary of the Prior Art
My co-pending application, Ser. No. 367,618, filed June 6, 1973, now U.S. Pat. No. 3,860,760, discloses several electronic techniques for automatically translating electrical signals from a first frequency band to a second frequency band using the principles of either time expansion or compression.
As stated in that application, which is hereby incorporated by reference, electronic time compression or expansion is useful in the processing and analyzing of electronic signals in the fields of radar, sonar, seismology, biomedicine, surgery, machine design, etc. Of course, there are advantages in such all-electronic systems but such systems are expensive to manufacture and difficult to maintain particularly if highly-skilled maintenance personnel are not available in the field.
For that reason, some effort has been expended to achieve similar time compression or expansion using rotating magnetic discs or endless magnetic tapes which, although they require moving parts, are considerably less complex from an electronic standpoint. For example, U.S. Pat. No. 3,621,150 which issued Nov. 16, 1971 to G. W. Poppas, discloses a technique for improving the intelligibility of the speech of a diver who is breathing a helium-oxygen atmosphere. More particularly, FIG. 3 of that patent discloses an apparatus wherein the diver's speech is recorded on an endless magnetic tape at a first speed and played back by a rotating magnetic head which is travelling with the tape but at a slower relative speed, thus shifting the speech signal to a lower, and hence more intelligible frequency band. The technique proposed by Poppas, however, is not suitable for digitilized or sampled signals nor is it suitable for the low frequency signals generated, for example, in moving target indicator (MTI) doppler radar systems.